I asked a friend who used to be a printer, and he suggested fleuron, but that seems to be a flower-shaped dingbat (aka flower, floret) that is used to separate text on a title page or used to make lines. Some books use an asterisk, an asterism (three asterisks in a triangular formation), or empty squares to set off text within a chapter after a change of time, place, or theme.

Ceci n'est pas un seing.